Strength of bump depends of object scale?
I think, it's a limitation of bump method. You can noted, that strength of bump effect not depends of scale, it depends of how big current face in world space...

So, use normal map. Normal map uses angles instead of depth, that's why objects, textured with normal map will looks the same in all scales.



Related: https://blenderartists.org/forum/showthread.php?359453-Cycles-displacement-corruption-when-scaling-an-object